1. Field of the Invention
This invention relates to calibration of instruments for automated analysis of liquid samples.
2. Discussion of the Art
Increasing public awareness of the need for the safety of the blood supply for transfusions has caused the transfusion industry to move toward the use of automated instruments, increased scrutiny by regulatory agencies, and centralization of blood bank testing. The “ABBOTT PRISM” system was conceived as an automated, high-volume, blood banking virology instrument that would reduce the need for highly trained instrument operators. According to the “ABBOTT PRISM” system, after the instrument automatically scans the samples during the loading process, the operator follows a simple set of menu-driven commands to initiate sample runs. The system can process 160 samples, or up to 960 tests, per hour. The only intervention required of the operator, with the exception of loading the racks with samples to be tested, involves adding commodities (e.g., pipette tips, reaction trays) and emptying two containers that hold solid waste. The system informs the operator when further action is required to continue processing samples. At the end of a batch of sample runs, the system can automatically print requested reports that include all the necessary information relating to the batch of sample runs for the laboratory.
Occasionally, the “ABBOTT PRISM” system requires the replacement of a liquid-dispensing component that is no longer operational. After the component is replaced, one must independently verify that the volume of liquid dispensed by the component falls within a specified range. A volume verification tool is used during maintenance procedures to check volumes of several liquid-dispensing mechanisms for liquids, such as, for example, sample probes, metering pumps for liquids, and the like. The volume verification tool uses an indirect measurement of capacitance to determine the height of a liquid in a containment vessel in order to determine the volume of fluid in the vessel. This measurement technique is subject to the effects of surface tension on the walls of the containment vessel. The meniscus of the fluid dispensed does not always uniformly adhere to the walls of the containment vessel, thereby causing the volume verification tool to fail the requirements for making a reading of the measurement, and, consequently, delay the user in completing the system checks required to restore the “ABBOTT PRISM” system to normal operation.
Users often find the volume verification tool difficult to use, thereby resulting in delays. In some cases, the completion of this verification procedure may require up to three days. Performance of the volume verification procedure can be improved marginally by certain techniques. These techniques include:                (1) placing the volume verification tool on a stable, level surface;        (2) replacing the containment vessels prior to each measurement rather than replacing the vessels after several uses;        (3) keeping the volume verification tool away from any air currents or sources of vibration;        (4) moving the hand-held probe away from the volume verification tool after fluid is introduced into the containment vessels; and        (5) using consistent manual fluid transfer techniques with a minimum amount of splashing.To minimize the down-time of the “ABBOTT PRISM” system and to simplify procedures for using the volume verification tool, it would be desirable to develop an alternative technique for measuring the volume of a liquid dispensed by a liquid-dispensing mechanism.        